Remove the ability for an event participant to have None hours earned

[bledsoef]

Low priority issue, leave for summer cohort

The EventParticipant model having a nullable hoursEarned field is old functionality leftover from when EventParticipant and EventRsvp were the same table. There really shouldn't be a case when their hours earned is None, at the very least it should only be 0. We need to change the following:

• The EventParticipant model's hoursEarned field
• Test data where hoursEarned is set to 0
• Tests that test for hoursEarned to be None (at least one is in test_spreadsheet
• Make sure there isn't any place in logic where we are setting their hoursEarned values

[bledsoef]

Remove the ability for an event participant to have None hours earned